ConnectionEstablished is called in an already established connection

Recently, discovery5 tests failed occasionally.
I read the failed test log and found that a connection established event is fired in the already existing connection. It made Foundry crash.

## Why this error found nowadays

Recently when we implementing a peer DB, we added an assertion that the same peer is not added twice in the DB. The assertion made Foundry panic.
It seems that the duplicated connection established event bug is aged, but recently found by the assertion.

## How should we fix it

The "connection established" event should not be called in an existing connection.

## ErrorLog

[2020_03_18T01_56_17_226Z.1.log](https://github.com/CodeChain-io/foundry/files/4346655/2020_03_18T01_56_17_226Z.1.log)
[2020_03_18T01_56_17_226Z.4.log](https://github.com/CodeChain-io/foundry/files/4346656/2020_03_18T01_56_17_226Z.4.log)